Percentages of cases and controls were not exactly identical on matching variables (birth year, gender, MCO) because we did not always get exactly 3 matched controls per case within each matching stratum.

↵b Recruitment of controls lagged behind case-children so that controls could be recruited to match case-children who had agreed to participate within birth year, gender, and MCO matching strata. The lagged recruitment meant that controls were an average of 3 months older than case-children at the time of interview/assessment.